The objective of this research will be to determine why intracisternal A particles (IAP) are present in preimplantation stages of the mouse. Experiments are proposed to distinguish between a fortuitous association and a necessary association, with IAP playing a specific developmental role. IAP may also be explained as a packaged gene product or may play a primarily oncogenic role. The immunologic relatedness of mouse embryo IAP to IAP from neoplastic tissues and to C particles will be ascertained. Experiments fall into 4 major categories: the characterization of mouse embryo IAP with anti-IAP serum and anti-C particle serum by means of indirect immunofluorescence and electron microscopy; the examination of wild mice for IAP and, if negative, correlated studies on infectivity and transmission; alteration of the embryonic regulation of IAP by inhibitors and inducers of oncornavirus formation; and the biochemical determination of IAP function within the embryo, including hybridization to test for the possible presence of a genomic IAP copy. Understanding the IAP-embryonic interaction may shed light on the action of oncornaviruses in general and is potentially of great value in the control of cancer. APPROACH: Immunological (fluorescence) and structural (EM) approaches are combined to assay the relatedness of IAP to oncornavirus and the success of inhibition and induction experiments. Biochemical approaches involve the synthesis of labeled products under the direction of IAP and hybridization. These are in collaboration with Dr. Apple.